Linear motion tables that are movable in orthogonal X, Y and Z axes (also called XYZ linear motion tables) have gained popularity in the machine tool and semiconductor industries due to the complexity of motion combinations demanded for the applications used in such industries. One common application is the use of actuators for driving an object, for example a semiconductor bond head which is attached to a support structure such as a table or platform. By controlling the position of the platform or table, the bond head may be positioned accordingly.
The simplest configuration for an XYZ linear motion system is with three overlying linear motion tables, the motion of each table being controlled by an actuator coupled to the table and serving to drive the table along one of the orthogonal motion axes. In this design, the weight of an actuator controlling an overlying table may generally be supported by another table since the actuator is designed to move with the other table. However the size as well as the weight of the actuator supported by another table and driven by another actuator are obstacles in obtaining high speed motion and position with extreme accuracy. When the loads on the motion axes are increased, various problems such as vibration of the structure arise so that complexity of the system may need to be increased to try to contain the problems. It is therefore desirable to reduce the load being carried or driven by each actuator.
In order to minimize the effect of increased weight on motion stability, several mechanisms which sought to decouple the dummy loads from the motion axes were developed. “Decoupling” in this sense means eliminating the interrelationship between one actuator and another so that the whole weight of one actuator is not carried or supported by the other actuator. Unlike tables that are only movable in the X and Y axes which only require one actuator to be at least partially decoupled from another table along an orthogonal motion axis, a prerequisite to having a decoupled XYZ table also involves the decoupling of a third actuator from another two motion axes.
In most conventional XYZ linear motion systems, at least two actuators are coupled to each other to a certain extent. Failure to decouple the various motion axes means that at least one table and actuator is required to take up the dummy load of another table and actuator. The additional load may well be insignificant if the actuators consist of solenoid, pneumatic cylinder motor or the like which have relatively smaller weights. However, with demand increasing for higher positioning accuracy and faster acceleration, the use of a linear motor having a heavy stator weight would cause greater concern.
A prior art apparatus which decouples a third actuator from a table driven by another two actuators is shown in FIG. 1. The three co-ordinate axes of a Cartesian system of co-ordinates are marked with X, Y, and Z, wherein the Z axis corresponds to a vertical direction. A compound table 44, which is driven by an X actuator 41 and a Y actuator 42, is movable along the two motion axes X and Y. These two motion axes can either be coupled or decoupled. A Z table 45 is guided relative to the compound table 44 by a pair of guidings 46 on the compound table 44 and is movable in the Z direction. A Z actuator 43 drives the Z table 45 to move in the Z direction through a contact 40 connected to an auxiliary coupler 47. This coupler 47, which is guided by a stationary rail 50, is movable in the Z direction. The contact 40 can be constructed with sliding material, a rolling ball or the like so that it can slide over a surface 48 of the Z table 45 while the compound table 44 moves along the X and Y directions. Permanent contact between the contact 40 and the surface 48 is ensured by the use of a tension spring 49, which is preloaded between the compound table 44 and the Z table 45. Thus, the Z actuator 43 can be mounted to a separate support and is decoupled from the motion tables 44, 45 while they are being driven by the X and Y actuators:
The actuators 41, 42 and 43 for these linear motion tables 44 and 45 may comprise mechanical, electrical or pneumatic means for providing linear motion. An end effector (not shown) mounted on the Z table 45 is capable of being moved in the X, Y and Z directions through a combination of movements of the respective motion tables 44, 45.
As compared to an XYZ table in a stacked configuration where the Z table is directly mounted on another table, the above XYZ table decouples the Z actuator 43 from the X and Y motion axes. Thus the dummy load due to the weight of the Z actuator mounted on another table can be avoided. However, the wearing out of the auxiliary coupler 47 as well as the surface 48 after prolonged use of the system may adversely affect both the leveling and positional accuracy of the XYZ table.
Planarity and smoothness for achieving positional and leveling accuracy of the Z table is an important factor for improving machining quality and to avoid backlash of the mating surfaces. However the wearing out of the above contacted parts is inevitable due to the friction produced between surfaces. Furthermore, the force from the anti-backlash or tension spring which keep the mating surfaces in permanent contact should be great enough, otherwise the table will oscillate due to slow response to actuation during fast acceleration. This causes an increase in the frictional forces which will expedite the wearing of the contacted parts.